physics 103: lecture 21 thermodyamics and car engines agenda for today:
TRANSCRIPT
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PHYSICS 103: Lecture 21
• Thermodyamics and Car Engines• Thermodyamics and Car Engines
Agenda for Today:
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LAWS OF THERMODYNAMICS: ZEROTH LAW
Heat flows from hot objects to cold objects until they have the same temperature.
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LAWS OF THERMODYNAMICS: FIRST LAW
Q = U + WQ = U + W
Conservation of Energy:
All energy entering a system must be conserved
IN
IN
UU
HEAT: Q
OUTOUT WORK: W = Fd
F d
Change in internal energy
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LAWS OF THERMODYNAMICS: FIRST LAW
Q = U + WQ = U + W
IN
IN
HEAT: Q
U: KE T
Example: Internal Energy &No Work Performed
IN
IN
HEAT: Q
OUTOUT WORK: W
Example: Internal Energy &Work Performed
U: KE T
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REAL ENGINES
Internal Combustion Engine: 4-stroke Otto cycle
Step 1: Intake of gas-air mixture from the carburetor
• Engine pulls piston out of cylinder
• Low pressure in cylinder
• Atmoshperic pressure pushes fuel air mixture into cylinder
• Engine does work on gases during this stroke
• Engine pulls piston out of cylinder
• Low pressure in cylinder
• Atmoshperic pressure pushes fuel air mixture into cylinder
• Engine does work on gases during this stroke
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LAWS OF THERMODYNAMICS: REAL ENGINES
Internal Combustion Engine: 4-stroke Otto cycle
Step 2: Compression of the air-gas mixture
• Engine pushes piston into cylinder
• High pressure and temperature in cylinder
•Engine does work on gases during this stroke
• Engine pushes piston into cylinder
• High pressure and temperature in cylinder
•Engine does work on gases during this stroke
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LAWS OF THERMODYNAMICS: REAL ENGINES
Internal Combustion Engine: 4-stroke Otto cycle
Step 3: Ignition and Expansion of the air-gas mixture
• Mixture burns to form hot gases
• Gases push piston out of cylinder
• As gas expands, pressure and temperature is lowered
• Gas does work on engine during this stroke
• Mixture burns to form hot gases
• Gases push piston out of cylinder
• As gas expands, pressure and temperature is lowered
• Gas does work on engine during this stroke
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LAWS OF THERMODYNAMICS: REAL ENGINES
Internal Combustion Engine: 4-stroke Otto cycle
Step 4: Exhaust of waste gas
• Engine pushes piston back into cylinder
• Pressure inside pushes burnt gas out of cylinder
• As gas expands, pressure and temperature is lowered
• Engine does work on gas during this stroke
• Engine pushes piston back into cylinder
• Pressure inside pushes burnt gas out of cylinder
• As gas expands, pressure and temperature is lowered
• Engine does work on gas during this stroke
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LAWS OF THERMODYNAMICS: REAL ENGINES
Internal Combustion Engine: 4-stroke Otto cycle
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LAWS OF THERMODYNAMICS: REAL ENGINES
Inline-4
V-6
Flat-4
Overhead Cam-4
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ENGINE EFFICIENCY
• Increase compression ratio => Get more work out of engine
• Limit to compression ratio: knocking - spontaneous ignition of
fuel/air mixture before the engine is ready to extract work
• High compression ratio cars use premium gasoline. Higher
octane ignites at higher temperatures
• On most normal cars, premium gas is a waste of money
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LAWS OF THERMODYNAMICS: Diesel ENGINES
• Uses compression to ignite fuel - squeezes air to high pressure and temperature (compression ratio of ~ 20:1 compared with 8:1 - 12:1) - injects fuel air in between compression and power strokes - fuel ignites spontaneously
• High compression allows for high efficiency
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Test Your Understanding
• How does the burned gas do more work on the piston during the
power stroke than the piston does on the unburned fuel-air
mixture during the compression stroke?
• Why is a car more likely to knock on a hot day than a cold day?
• High compression ratio cars use premium gasoline. Higher
octane ignites at higher temperatures
• On most normal cars, premium gas is a waste of money
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Main Points from Today’s Lecture
• Engines• EnginesYou should understand that some of the heat (that flows from hot objects to cold objects) can be used to do work. Any device that does this is called a heat engine. You should have a basic understanding of the strokes of a car engine.